There isn't.
Look at it this way: Let's say you decide to measure cooktemps by suspending or otherwise fixing a probe just above whatever meat your cooking. (Let's say it's ribs.) You follow your procedure of choice, you test the ribs for tender, pull them when done, and they're great. Let's say the cooktemp was ~ 260 the whole time.
You repeat your approach several times with excellent results.
On a whim you decide to check lid temp. It's, say, 225 -- or 310 -- doesn't matter. And, well, it really won't matter because you have an approach that works for you -- and that has worked for you repeatedly. That the lid temp is different means nothing because you are not cooking with that in mind.
(All this is true were you to have chosen the lid to work off of at the beginning, as well.)
But, let's say, you are new to the WSM. You read the boards and want to cook ribs and based on what you read you decide to use 260 as a target. If you then check both lid and grate temps and see a disparity you might become frustrated because your focus is on honing in on this particular temp. (This happens all the time -- just see the many temp-frustration posts here and elsewhere.) You might place all your energy into figuring out the disparity, or trying to 'fix' the problem -- trying to square the circle.
The energy, instead, should simply be placed on trying to cook good ribs and, as long as your cooker is hot enough (>225) but not too hot (<350) this is not difficult at all. If you get used to cooking within a particular range the actual number really doesn't matter. E.g., your therm (in the first example of repeated success) could simply have the numbers 1-10 on the dial, no actual temp numbers. If you cooked at, say, 3, and focused on a successful cook, achieved it, and repeated it many times, you wouldn't even know what temps you were actually cooking at -- and it wouldn't matter. It's where you place your focus that counts.
Hand-in-hand with this is abandoning the notion of specific cook times. Where new cooks get into trouble (frustration-wise) is by either an over-emphasis on hitting a specific temp number (and freaking out if spikes occur), or cooking something at X temp for Y number of hours and, if unsuccessful (as is often the case), wondering why their meat was chewy, tough, or dry.
Numerous cooktemp ranges are possible. Cook time is based on many variables and only serves (often rather loosely) as a guide. If the focus is as it should be -- cooking the meat till moist and tender -- success is more easily achieved because the only way to really tell if the meat is going to be moist and tender is by checking the meat to see if it is -- not pulling it off when it reaches X internal temp, or when it has cooked for Y number of hours. (Once you know how meat should feel you can cook at any reasonabble temp --even temps you are not used to cooking at -- and achieve great results every time.)
[The reason I use lid temp is because it involves nothing more than sticking a silicone-plugged therm in a vent hole. On occasion, when I feel the need to monitor remotely (it doesn't happen much), I will dangle a probe through a vent hole. One can use grate temps instead -- but one should be careful of probe placement. Too close to the edge of the grate is a problem. Too close to the meat is a problem -- especially if the cut is large. There are good, easy techniques to use if you would prefer to monitor grate temps (I simply don't want to bother -- I rarely monitor internal temps either) like attaching your grate probe to your internal probe, etc. But, the point: pick one or the other for now, choose a temp range, then focus on the meat.]
) And finally, if this is something I/we have to deal with...which one should we focus more on? I'm guessing the grate temp?
